In recent years, global warming trend has become more pronounced. Efforts are being made to introduce electric vehicles which use absolutely no fossil fuel as well as hybrid vehicles such as hybrid cars powered by some fossil fuels, in order to reduce emissions of carbon dioxide, a greenhouse gas, as part of measures to the global warming.
Regardless of the fuel being used, moisture condensation occurs on the glass windows during periods such as in winter when the outside air temperature is lower than the inside temperature. Such condensation can obstruct the driver's view and interfere with safety in driving, and electric vehicles are no exception. Water loss (insensible perspiration) of a passenger would be a cause of this condensation problem.
Dehumidification of the air inside the passenger compartment is a measure against the problem in question. However, cooling/dehumidification using a compression refrigeration system as those employed in traditional vehicles is not suitable for the electric or hybrid vehicles. Consumption of a significant amount of electricity to heat the cold air causes a secondary problem of a decrease in running mileage.
An example is given now. It is assumed that three passengers get in an electric vehicle having an interior volume of 4 m3 (=air weight of 4.8 kg) at an outside temperature of 5° C., 60% RH (absolute humidity of 2.6 g/kg). Water lost through insensible perspiration is estimated to be 30 g/h per person for adults, so that the absolute humidity increases by 18.8 g/kg per hour in the interior of the vehicle. When the air temperature near the glass windows is 5° C., it takes about 9 minutes to reach the relative humidity of 100% (absolute humidity of 5.4 g/kg), at which point condensation (fogging of the windows) begins.
Traditional vehicles use the heat that the engine dissipates and is passed through a heater warming the air near the glass windows to reduce the relative humidity of the air, thereby avoiding condensation. However, this approach cannot be applied to the electric vehicles having no effective hear source.
Consuming the electric power for heating to raise the temperature of the passenger compartment or to dissipate the fog results in a shorter running mileage of the electric or hybrid vehicle and a larger lithium ion battery which is rather expensive. Such a situation is undesirable in the economic considerations, so that quick development of an anti-fog and HVAC system that uses less electricity is required.
Recently, desiccant humidity control has been proposed as an effective means for air dehumidification. For example, Patent Document 1 shows an HVAC system in which two desiccants (dehumidifying agents placed in a desiccant wheel) are arranged so that they can rotate between a process air path and a regeneration air path. When one desiccant accepts moisture from the process airstream, the other desiccant is regenerated by passing the regeneration air through. Two desiccants are alternately used to dehumidify the outdoor air to be introduced into the passenger compartment.